Various types of marine cargo transfer devices have heretofore been proposed for loading a marine vessel, either from another vessel or from a shore facility, with fuel oil, crude oil, or other types of fluid cargo. Because of the shear weight and volume of the fluids being transferred, such marine loading equipment is necessarily large and difficult to handle. Particularly in transferring fluid cargoes at sea between two floating vessels, both of which are subject to motions of heave, roll, pitch, and yaw, as well as moving toward and away from each other under the action of wind and wave, severe strains can be placed on such cargo transfer equipment. It has been the practice, particularly in transferring fluids between two floating vessels, to utilize flexible hoses, the weight being supported by cranes or other types of multiple pulley and cable arrangements from one or both of the vessels. Flexible hoses allow for relative motions between the two vessels. Such arrangements have not proved practical for transferring petroleun products that are liquefied at low temperatures, such as propane which must be maintained at temperatures below -50.degree. F, or a liquefied natural gas which must be maintained at cryogenic temperatures. Normal hose materials are not suitable for such low operating temperatures. While special flexible hoses have been provided for conducting very low temperature fluids, such hoses do not have the strength and durability to operate under the severe conditions encountered at sea.
Marine loading arms have been developed which utilize rigid pipe sections coupled by swivel joints, such as described, for example, in U.S. Pat. No. RE 25,855. By using rigid pipe sections, they can be made of materials, such as stainless steel, which retain their strength at low temperatures. However, such cargo transfer devices have to been suitable for transfer between vessels on the high seas, but have been restricted to dock side loading where the arms have been permanently mounted on the dock and remain relatively stationary. The only motion of the vessel is the change in draft with loading and unloading of the cargo. To make the loading arms more maneuverable, the arms have been counterbalanced by weights so that the arms remain substantially statically balanced in all operating positions.
However, such rigid counterbalanced arms are not suitable for use in transferring fluid cargo between two floating vessels where inertial forces due to the continuous movement of the two vessels as they react to wind and wave conditions are substantial. In the past, it has been the practice to use the fluid transporting pipes themselves as the structural members to reduce the total weight of the arms and counterbalancing weights. However, such an arrangement puts undue strain on the swivel joints which interconnect the pipe sections. To use a supporting structure for the pipe to transfer the load to the base greatly increases the amount of mass required to counterbalance the structure. The greater the mass of the structure, the greater the inertial forces which are generated by the motions of the vessels.